Phase-Partitioning Principles, System Differences, and Application Selection in Phenol-Based DNA and RNA Extraction
Phase-Partitioning Principles, System Differences, and Application Selection in Phenol-Based DNA and RNA Extraction
Phenol-based nucleic acid extraction separates DNA or RNA from proteins, lipids, and cellular debris through sample lysis, protein denaturation, and aqueous-organic phase partitioning. Neutral or mildly alkaline phenol systems are generally used for DNA and total nucleic acid extraction, whereas acidic guanidinium-phenol systems are mainly used for selective RNA extraction. The pH of the phenol reagent, lysis-buffer composition, sample loading, and interphase-transfer technique jointly affect nucleic acid yield, purity, and integrity.
Keywords: phenol-based extraction; DNA extraction; RNA extraction; phenol-chloroform; isoamyl alcohol; acidic phenol; guanidinium thiocyanate; liquid-liquid partitioning; nucleic acid purification; quality control
1 Basic Principles and Technical Characteristics of Phenol-Based Nucleic Acid Extraction
1.1 Sample Lysis and Nucleic Acid Release
Within cells, DNA and RNA form complexes with histones, ribosomal proteins, and other nucleic acid-binding proteins. Extraction therefore requires disruption of the plasma membrane, organelle membranes, or cell wall and dissociation of nucleoprotein complexes. DNA extraction commonly uses detergents, EDTA, and proteinase K for lysis and protein digestion. RNA extraction usually employs strong denaturants such as guanidinium thiocyanate to rapidly inactivate RNases while lysing the sample. Incomplete lysis reduces nucleic acid recovery, whereas excessively vigorous mechanical treatment can shear high-molecular-weight DNA.
1.2 Protein Denaturation and Nucleic Acid Partitioning
Phenol disrupts hydrogen bonds and hydrophobic interactions within proteins, causing loss of native conformation and reducing protein solubility in the aqueous phase. After centrifugation and phase separation, nucleic acids partition into the aqueous phase or interphase according to system pH, salt concentration, and molecular properties. Denatured proteins and cellular debris mainly accumulate at the interphase, whereas lipids and other hydrophobic substances enter the organic phase.
1.3 Technical Characteristics of Phenol-Based Extraction
Phenol-based methods provide strong removal of proteins and lipids and are suitable for high-protein, high-lipid, or compositionally complex samples. They can also be used to extract high-molecular-weight genomic DNA and total RNA. Their main limitations are the relatively large number of operating steps, the risk of interphase contamination during aqueous-phase transfer, and the potential inhibition of PCR, reverse transcription, restriction digestion, ligation, and sequencing-library construction by residual phenol, chloroform, guanidinium salts, or ethanol. Phenol and chloroform are toxic and volatile and should be handled under ventilated conditions with waste disposed of separately.
2 Roles of Phenol, Chloroform, and Isoamyl Alcohol in Liquid-Liquid Phase Separation
2.1 Protein-Denaturing Action of Phenol
Phenol is the principal protein-denaturing component of the phase-partitioning system. It unfolds histones, membrane proteins, and nucleases and drives them into the organic phase or interphase, thereby releasing protein-bound nucleic acids. Prolonged exposure of phenol to air and light may cause oxidation, and oxidation products can increase the risk of nucleic acid damage and inhibition of downstream reactions. Phenol used for nucleic acid extraction should therefore be protected from light, tightly sealed, and stored under specified conditions.
2.2 Auxiliary Roles of Chloroform and Isoamyl Alcohol
Chloroform increases the density of the organic phase, promotes formation of a clear interface between the aqueous and organic phases, and enhances partitioning of lipids and hydrophobic contaminants into the organic phase. The aqueous phase obtained after phenol extraction can be treated again with chloroform to reduce residual phenol.
Isoamyl alcohol is mainly used to reduce foaming and emulsification and improve phase stability. A common phenol-chloroform-isoamyl alcohol volume ratio is 25:24:1, whereas chloroform-isoamyl alcohol mixtures commonly use 24:1. The actual ratio should follow the specific protocol and reagent formulation.
2.3 Phase-Separation Results and Aqueous-Phase Transfer
After centrifugation, the upper aqueous phase mainly contains nucleic acids and water-soluble components, the lower organic phase mainly contains phenol, chloroform, lipids, and hydrophobic substances, and the interphase is enriched in denatured proteins, cellular debris, and part of the nucleic acid content. During aqueous-phase transfer, liquid should be aspirated slowly from the surface, leaving a small volume above the interphase. Aspirating too deeply may increase apparent recovery but also increases contamination by proteins, genomic DNA, and organic reagents.
3 Effects of Phenol-Reagent pH on DNA and RNA Partitioning
3.1 Neutral or Mildly Alkaline Phenol Systems
In neutral or mildly alkaline systems at approximately pH 7.5-8.0, both DNA and RNA generally retain good water solubility and mainly partition into the upper aqueous phase, whereas proteins distribute into the interphase and organic phase. Neutral or mildly alkaline phenol is therefore commonly used for genomic DNA, plasmid DNA, and total nucleic acid extraction. When DNA is the target, coextracted RNA in the aqueous phase usually requires removal by RNase treatment.
3.2 Acidic Phenol Systems
Acidic phenol is generally combined with guanidinium thiocyanate and chloroform to form an RNA-extraction system. Under acidic and high-ionic-strength conditions, RNA is mainly retained in the aqueous phase, high-molecular-weight DNA partitions more strongly into the interphase and organic phase, and proteins are extensively denatured by guanidinium salts and phenol. Acidic phase separation does not completely eliminate DNA. Short DNA fragments, sample overloading, or disturbance of the interphase may still cause genomic DNA to enter the aqueous phase.
3.3 Combined Factors Affecting Nucleic Acid Partitioning
DNA and RNA partitioning is not determined by pH alone but is also influenced by nucleic acid length, conformation, hydration state, salt concentration, guanidinium content, and sample protein composition. Using acidic RNA-extraction phenol for genomic DNA extraction may reduce DNA recovery, whereas using neutral DNA-extraction phenol for RNA extraction can increase DNA coextraction. The buffering state of the phenol reagent should therefore match the target nucleic acid and extraction system.
4 Composition, Workflow, and Critical Control of Phenol-Based DNA Extraction Systems
4.1 Composition of DNA Extraction Systems
Phenol-based DNA extraction systems generally contain Tris buffer, EDTA, detergents, proteinase K, neutral or mildly alkaline phenol, chloroform, and isoamyl alcohol. Tris maintains an appropriate pH, EDTA chelates Mg²⁺ and Ca²⁺ to inhibit nucleases, detergents disrupt cell membranes and denature proteins, and proteinase K further digests histones, membrane proteins, and nucleases.
4.2 Workflow of Phenol-Based DNA Extraction
After the sample has been completely lysed in lysis buffer and proteinase K digestion is complete, neutral phenol or a phenol-chloroform-isoamyl alcohol mixture is added. The sample is mixed gently and centrifuged for phase separation. The upper aqueous phase is transferred to a new container and may be extracted again with chloroform-isoamyl alcohol to remove residual phenol.
DNA in the aqueous phase is then precipitated with salt and ethanol or isopropanol, washed with 70%-75% ethanol, moderately dried, and redissolved in TE buffer or nuclease-free water.
4.3 Integrity Control of High-Molecular-Weight DNA
When extracting high-molecular-weight genomic DNA, high-speed vortexing, repeated pipetting, and narrow-bore pipette tips should be minimized to reduce mechanical shearing. Viscous lysates should be mixed by slow inversion. Increasing the number of extraction steps can improve protein removal but also increases DNA shearing and aqueous-phase transfer losses. The number of extractions should therefore be adjusted according to sample protein content.
4.4 Common Problems in DNA Extraction
Low DNA yield is generally associated with incomplete sample lysis, loss during aqueous-phase transfer, or incomplete alcohol precipitation. A low A260/A280 ratio usually indicates protein or phenol contamination, whereas a low A260/A230 ratio may be associated with residual phenol, salts, EDTA, or ethanol. If RNA contamination in the DNA sample is excessive, RNase A treatment can be performed, followed by repurification according to the downstream application.
5 Composition, Workflow, and Critical Control of Phenol-Based RNA Extraction Systems
5.1 Composition of RNA Extraction Systems
Phenol-based RNA extraction commonly uses a strongly denaturing system containing guanidinium thiocyanate, acidic phenol, and chloroform. Guanidinium thiocyanate disrupts cellular structures, dissociates nucleoprotein complexes, and rapidly inactivates RNases. Acidic phenol and chloroform promote separation of RNA from DNA, proteins, and lipids. Some systems also contain acetate, surfactants, or reducing agents to maintain acidic conditions and enhance RNase inactivation.
5.2 Workflow of Phenol-Based RNA Extraction
Samples should be immediately added to a sufficient volume of lysis reagent and thoroughly homogenized so that tissues are completely dispersed and RNases are rapidly inactivated. After chloroform is added to the lysate, the mixture is thoroughly mixed and centrifuged for phase separation. The upper aqueous phase contains RNA, whereas the interphase and organic phase mainly contain DNA, proteins, and lipids.
The aqueous phase is transferred to a new container, and RNA is precipitated with isopropanol or ethanol. The precipitate is washed with ethanol, moderately dried, and redissolved in RNase-free water.
5.3 RNase Contamination and Sample Storage
RNA extraction should use RNase-free consumables, solutions, and working environments. After collection, samples should be immediately lysed, frozen in liquid nitrogen, or preserved with an RNA-stabilizing reagent. Delayed processing, insufficient lysis reagent, repeated freeze-thaw cycles, and contaminated equipment can all cause RNA degradation. Extracted RNA should preferably be aliquoted and stored at low temperature to avoid repeated freezing and thawing.
5.4 DNA Contamination in RNA Extraction
Genomic DNA contamination in RNA samples commonly results from sample overloading, insufficient homogenization, overly deep aspiration of the aqueous phase, or disturbance of the interphase. RNA intended for reverse-transcription quantitative PCR or transcriptome analysis generally requires DNase I treatment, and a no-reverse-transcription control should be used to assess residual DNA. After DNase treatment, the enzyme, metal ions, and reaction buffer should also be removed.
5.5 RNA Precipitation and Redissolution
Low-concentration RNA is easily lost during alcohol precipitation. Recovery can be improved by extending the precipitation time or adding an appropriate coprecipitant. Insufficient ethanol washing leaves residual guanidinium salts and other salts, whereas excessive drying makes RNA difficult to redissolve. RNA should be redissolved with gentle mixing and without prolonged high-temperature treatment.
6 Methodological Differences and Selection Criteria Between DNA- and RNA-Extraction Phenol Systems
6.1 Major Differences Between Phenol-Based DNA and RNA Extraction
Comparison Item | Phenol-Based DNA Extraction | Phenol-Based RNA Extraction |
Phenol-reagent conditions | Neutral or mildly alkaline | Acidic |
Major lysis components | Detergents, EDTA, and proteinase K | Guanidinium thiocyanate, acidic phenol, and reducing components |
Distribution of target nucleic acid | DNA mainly enters the aqueous phase | RNA mainly enters the aqueous phase |
Major components removed | Proteins, lipids, and cellular debris | DNA, proteins, lipids, and RNases |
Operational focus | Minimize shearing of long DNA molecules | Rapidly inactivate RNases and avoid interphase contamination |
Common contaminants | RNA, proteins, and residual phenol | Genomic DNA, guanidinium salts, and residual phenol |
Subsequent nuclease treatment | RNase A is commonly used | DNase I is commonly used |
Major applications | Genomic DNA, plasmid DNA, and total nucleic acids | Total RNA and some small RNAs |
6.2 Selection of the Extraction System
When high-molecular-weight genomic DNA is required for long-fragment amplification or long-read sequencing, a neutral or mildly alkaline phenol system should be selected and mechanical shearing minimized. When total RNA is required for reverse-transcription quantitative PCR, transcriptome sequencing, or Northern blotting, an acidic guanidinium-phenol system should be selected.
When DNA, RNA, and proteins need to be separately recovered from the same sample, a multicomponent phase-partitioning extraction system can be used. Samples with low starting amounts, high-throughput requirements, or automation needs are generally more suitable for column- or magnetic bead-based methods.
7 Adjustment of Phenol-Based Extraction Conditions for Different Samples
7.1 Cultured Cells and Animal Tissues
Cultured cells can be lysed directly by adding lysis buffer, whereas animal tissues require mechanical homogenization according to tissue mass and structure. Adipose tissue readily forms lipid and emulsion layers, so the starting amount should be reduced and the volume of lysis reagent or organic phase increased. Muscle, skin, and connective tissue may require prolonged proteinase K digestion or more extensive homogenization to improve lysis.
7.2 Blood, Plasma, and Low-Input Samples
Whole-blood DNA extraction requires removal of hemoglobin, plasma proteins, and anticoagulants. Red blood cell lysis or leukocyte enrichment may be performed when necessary. Nucleic acid concentrations in plasma, serum, and small numbers of cells are low, and losses during aqueous-phase transfer and alcohol precipitation account for a larger proportion of total recovery. Coprecipitants such as glycogen can be added, and unnecessary transfer steps should be minimized.
7.3 Bacteria, Fungi, and Yeast
Gram-positive bacteria, fungi, and yeast have thick cell walls and require lysozyme, lytic enzymes, glass beads, or mechanical grinding to assist disruption. Microbial RNA extraction should enter a strongly denaturing guanidinium-phenol system immediately after cell-wall disruption to minimize RNA degradation and changes in transcriptional state. When extracting high-molecular-weight bacterial genomic DNA, excessive agitation should be avoided to prevent shearing.
7.4 Plant Tissues
Plant tissues contain polysaccharides, polyphenols, pigments, and cell-wall components that can coprecipitate with nucleic acids and inhibit downstream enzymatic reactions. High-salt or CTAB systems can be used for polysaccharide-rich samples, whereas polyvinylpyrrolidone and reducing agents can be added to polyphenol-rich samples. Plant RNA extraction generally requires thorough grinding under liquid-nitrogen conditions followed by rapid addition of guanidinium-phenol lysis reagent.
7.5 Fixed Tissues and Degraded Samples
Nucleic acids in formalin-fixed paraffin-embedded tissues may be cross-linked, fragmented, and chemically modified. Deparaffinization, protease digestion, and reversal of cross-links are required before purification. Phenol extraction can remove proteins and certain contaminants but cannot restore already fragmented nucleic acids. Short-amplicon PCR, targeted sequencing, or library-construction methods suitable for degraded RNA should therefore be selected according to fragment length.
8 Residual Phenol, Nucleic Acid Degradation, and Extraction Quality Evaluation
8.1 Nucleic Acid Concentration and Purity
Nucleic acid concentration can be measured by ultraviolet absorbance or fluorescence-based assays. UV methods are convenient, but phenol, free nucleotides, and other absorbing substances may cause concentration overestimation. Fluorescence-based methods provide greater selectivity for DNA or RNA and are more suitable for low-concentration or complex samples.
The A260/A280 ratio of pure DNA is generally close to 1.8, whereas that of pure RNA is generally close to 2.0. An A260/A230 ratio of approximately 2.0-2.2 is generally desirable. However, absorbance ratios fluctuate substantially in low-concentration samples and should not be used as the sole criterion for quality assessment.
8.2 DNA and RNA Integrity
Genomic DNA integrity can be evaluated by agarose gel electrophoresis or fragment analysis. High-molecular-weight DNA should mainly appear as a concentrated signal in the high-molecular-weight region, whereas pronounced smearing suggests mechanical shearing or nuclease degradation.
RNA integrity can be assessed using ribosomal RNA bands, electrophoretic profiles, RNA integrity numbers, or DV200. RNA degradation reduces reverse-transcription efficiency and affects transcriptome coverage and expression quantification.
8.3 Residual Organic Reagents and Salts
Residual phenol affects A260/A280 and A260/A230 ratios and inhibits polymerases, reverse transcriptases, ligases, and restriction endonucleases. It can be removed by an additional chloroform extraction or secondary purification using columns or magnetic beads. Residual guanidinium salts and ethanol usually result from insufficient washing or inadequate drying of the precipitate and can also markedly inhibit downstream enzymatic reactions.
8.4 Downstream Functional Validation
DNA intended for PCR should be evaluated for amplification efficiency and nonspecific products. DNA intended for restriction digestion, cloning, or sequencing should be validated for digestion, ligation, and library-construction performance.
RNA samples should be evaluated using reference-gene amplification, reverse-transcription efficiency, and no-reverse-transcription controls to assess DNA contamination and reaction inhibition. Extraction blanks should also be included to identify nucleic acid contamination introduced by reagents, consumables, or the environment.
9 Products Related to Phenol-Based DNA and RNA Extraction
9.1 DNA and RNA Extraction and Auxiliary Products
Catalog # | Product Name | Grade & Purity | Main Application |
DNA Phenol Reagent | BioReagent, Suitable for molecular biology | Provides a phenol phase suitable for DNA extraction and is used for protein denaturation and separation of DNA from proteins | |
RNA Phenol Reagent | BioReagent, Suitable for molecular biology | Provides a phenol phase suitable for RNA extraction and is used for protein denaturation and separation of RNA from DNA and proteins | |
AllPure DNA/RNA/Protein Kit | — | Sequential recovery of DNA, RNA, and proteins from the same sample for multicomponent analysis | |
Trizol | BioReagent, ready-to-use, Suitable for molecular biology, RNase-free, for DNA and RNA applications | Cell and tissue lysis, RNase inactivation, and total RNA extraction | |
FFPE DNA/RNA Kit | — | Release and purification of DNA and RNA from fixed tissues | |
Polysaccharide & Polyphenol Rich Plants Total RNA Extraction Kit (DNase I) | BioReagent, Suitable for molecular biology | Total RNA extraction and genomic DNA removal from plant samples rich in polysaccharides and polyphenols | |
Micro Sample Total RNA Rapid Extraction Kit | BioReagent, Suitable for molecular biology | Total RNA extraction from low-input cells, tissues, or body-fluid samples | |
RNase-Free Water | 100 mL | Redissolution of RNA precipitates, preparation of RNase-free buffers, and molecular biology reactions | |
Proteinase K Solution (20 mg/mL) | BioReagent, DNase- and RNase-free, Suitable for molecular biology, sterile, ≥95% (Native-PAGE), 20 mg/mL | Digestion of histones, nucleases, and other proteins during DNA extraction | |
CTAB Precipitation Solution | BioReagent, Suitable for molecular biology, for DNA and RNA applications | Nucleic acid extraction and polysaccharide removal from plants, fungi, and high-polysaccharide samples | |
Glycogen (for nucleic acid precipitation, 20 mg/mL) | BioReagent, DNase- and RNase-free, Suitable for molecular biology, for DNA and RNA applications, 20 mg/mL | Nucleic acid coprecipitant used to improve alcohol-precipitation recovery of low-concentration DNA or RNA | |
DNase I-ST | Recombinant, Suitable for molecular biology, EnzymoPure™, RNase-free, ≥90% (SDS-PAGE), 2 U/μL | Removal of residual genomic DNA from RNA samples | |
RNase A | Bioactive, ActiBioPure™, DNase-free, native, high-performance, Suitable for molecular biology, EnzymoPure™, ≥85% (SDS-PAGE) | Removal of RNA contamination from DNA samples | |
RNase and DNase Away | BioReagent, ready-to-use | Control of RNase and DNase contamination on laboratory benches, instruments, and working areas |
9.2 Chemical Reagents Related to Phenol-Based Nucleic Acid Extraction
Product Name | CAS No. | Main Application |
Phenol | Protein denaturation and formation of the organic phase for separation of nucleic acids from proteins | |
Isoamyl alcohol | Reduces foaming and emulsification and improves phase separation in phenol-chloroform systems | |
Guanidinium thiocyanate | Cell lysis, protein denaturation, and rapid RNase inactivation | |
Sodium dodecyl sulfate | Disrupts cell membranes and denatures proteins | |
Ethylenediaminetetraacetic acid | Chelates divalent metal ions and inhibits nuclease activity | |
Tris(hydroxymethyl)aminomethane | Preparation of lysis, equilibration, and redissolution buffers and maintenance of system pH | |
Sodium acetate | Provides cations for alcohol precipitation of nucleic acids and adjusts ionic strength | |
Sodium chloride | Adjusts ionic strength and promotes nucleic acid precipitation or polysaccharide separation | |
Isopropanol | Rapid precipitation of DNA and RNA | |
Cetyltrimethylammonium bromide | Lysis and polysaccharide removal from plant, fungal, and high-polysaccharide samples | |
Polyvinylpyrrolidone | Adsorption of polyphenols in plant samples and reduction of interference from oxidation products | |
Dithiothreitol | Reduction of protein disulfide bonds and auxiliary inhibition of RNase activity |
Phenol-based DNA and RNA extraction systems should be selected according to the target nucleic acid, sample characteristics, and downstream application, with particular attention to phenol-reagent pH, interphase transfer, and residual organic reagents.
For more related articles, please see below:
[1] The Golden Pair for Nucleic Acid Extraction: RNase A and Proteinase K
[2] GITC——Key Reagent for Nucleic Acid Extraction and Sample Protection